Rechargeable battery cells with solid electrodes are of two general types: (1) open or vented, sometimes referred to as "flooded"; and (2) sealed, commonly referred to as "starved". Generally, in a flooded cell, the electrodes are immersed in electrolyte, while in a starved cell, the electrodes are not immersed in electrolyte. These two types of batteries differ primarily in the way in which they deal with gases, namely oxygen and/or hydrogen, which are generated in the battery cells toward the end of the charging operation and during overcharging.
Type 1 allows the gases to vent to the atmosphere; in Type 2 the gases (primarily oxygen) are recombined back into water inside the sealed battery cell. Type 2 is preferred from the user's point of view because the sealed cell requires no periodic maintenance, maintains charge balance between the plates, can operate in any position, releases no explosive gases and does not leak corrosive chemicals into the environment.
Two different kinds of sealed cells (Type 2) are known in the industry. One is a standard design common in consumer cylindrical and small prismatic or rectangular cells up to about 50 Ah capacity for Ni-Cd, and about 500 Ah for Pb-Acid. The other (which has been commercialized for Ni-Cd batteries only) employs recombination plates and uses a split negative plate and is available in up to about 100 Ah capacity. Although different in construction and performance, both of these sealed cells share some fundamental principles, as follows.
1. They attempt to minimize hydrogen evolution by using an excess of discharged negative material and rely on the oxygen cycle to maintain discharged negative material in the cell at all times. PA0 2. They are limited to individual vessel designs, that is, individual cells each hermetically sealed to ensure that all oxygen generated in a particular cell will recombine in the same cell (with some exception for monoblock Pb-Acid that sometimes use common gas space). PA0 3. They use starved electrolyte in the stack of electrodes and separators to permit oxygen transport to the negative electrode. This dictates tight stacking of electrodes, small interelectrode distance and close control of the electrolyte level in the cell. PA0 4. If placed in a multiple cell battery, they require close matching of cell capacities, charge efficiencies and temperatures to guarantee long life and avoid cell reversal, hydrogen evolution, overpressure and overheating.
Vented cells, on the other hand, are more robust. They do not require as tight a control in manufacturing, they are less sensitive to overcharge and overdischarge or deep discharge, and there is less concern with cell temperature and pressure. They are generally less expensive to build and more applicable to large cells as well as large batteries, yet they pose considerable difficulties to the user who is concerned with periodic maintenance, explosive gas releases into the environment, electrolyte splashing, and loss of plate balance in the cells.
The above discussion indicates the need for a battery that is sealed and requires no maintenance, yet is more robust in design, manufacturing and use, more applicable to large cells and large multi-cell batteries, easy to produce at economical costs and offers advantages in energy density.
A prior U.S. Patent which tried to address some of these problems was U.S. Pat. No. 5,143,799, which is hereby incorporated in its entirety by reference herein. This patent disclosed a sealed rechargeable nickel zinc or silver zinc cell which was divided into two compartments, one having a zinc electrode and a first hydrogen electrode and a second having a nickel or silver electrode and a second hydrogen electrode electrically connected to the first hydrogen electrode. A common gas space was provided for the two compartments so that the hydrogen and oxygen gases could recombine to water and the container could be sealed. Among other expensive features, this battery requires a hydrogen electrode in each cell, which is very costly, and the cells need to be starved.